A photodetector in a conventional optical head includes a light receiving part configured from a silicon semiconductor (multilayer silicon wafer), an operation circuit which converts the amount of luminous flux received by the light receiving part into voltage and performs a predetermined operation, and a signal output unit which outputs the voltage to become an output signal from the operation circuit. In addition, a circuit board and a pad part configured on the circuit board are connected via wire bonding. Moreover, the light receiving part, the operation circuit, the signal output unit and the circuit board are packaged and retained with a resin package.
Note that a terminal part that is electrically connected to the pad part is configured on the circuit board, and the terminal part is mounted on and fixed to a flexible printed circuit board (hereinafter also referred to as the “FPC board”). In addition, the photodetector is fixed to a holder, and the holder is bonded and fixed to an optical base (for example, refer to Patent Literature 1). Here, the resin package has a thermal resistance; that is, it has a low coefficient of thermal conductivity. Thus, regardless of whether the material of the holder is metal or resin, the amount of heat that transfers from the photodetector to the resin package is extremely small. Meanwhile, since the photodetector itself has a large volume and a large thermal capacity, the temperature rise of the photodetector caused by the amount of heat that is generated from the photodetector itself was approximately 10° C. or less.
FIG. 25 is a diagram showing the configuration the optical system of a conventional optical head 400. In FIG. 25, the optical head 400 comprises a semiconductor laser 401, a diffraction grating 402, a beam splitter 403, a collimator lens 404, an objective lens 405, an objective lens actuator 406, a cylindrical lens 408, a photodetector 409 and a holder 410.
The light beam emitted from the semiconductor laser 401 is separated into a plurality of different luminous fluxes by the diffraction grating 402. The luminous flux transmitted through the diffraction grating 402 is reflected by the beam splitter 403, and converted into a parallel luminous flux by the collimator lens 404. In addition, the luminous flux enters the objective lens 405 and becomes so-called 3-beam convergent light. This convergent light is irradiated to the recording layer of the optical disk 407. The luminous flux that has been reflected and diffracted by the recording layer of the optical disk 407 is once again transmitted through the objective lens 405 and is then transmitted through the beam splitter 403. The luminous flux transmitted through the beam splitter 403 passes through the cylindrical lens 408 and enters the photodetector 409.
Here, the photodetector 409 is fixed to the holder 410, and receives the luminous flux that passed through the holder 410. FIG. 26 is a diagram showing the arrangement of the light receiving parts on the light receiving surface of the photodetector of a conventional optical head. The luminous flux transmitted through the cylindrical lens 408 is received by the quartered light receiving part 420, and a so-called focus error signal is thereby generated.
FIG. 27 is a diagram showing the detection optical system of a conventional optical head, and FIG. 28 is a diagram showing the luminous flux that is formed at the quartered light receiving part of the photodetector of a conventional optical head. As shown in FIG. 27, the cylindrical lens 408 includes a cylindrical surface 408a on the exit face side of the luminous flux, and includes a concave lens surface 408b on the entry face side. The cylindrical lens 408 generates an astigmatic difference having a different focal point position at a 90-degree angle in the in-plane that is orthogonal to the optical axis. Moreover, the direction of the cylindrical surface 408a is disposed at an angle that is inclined substantially 45 degrees relative to the quartered light receiving part 420 of the photodetector 409.
The relative distance between the recording layer of the optical disk 407 and the objective lens 405 changes due to the face deflection of the optical disk 407 and other reasons. Consequently, as shown in FIG. 28, the luminous flux 422a at the focal point position becomes a circular shape, and the luminous flux 422b at the anterior focal line and the luminous flux 422c at the posterior focal line become a mutually orthogonal oval shape.
In FIG. 26, the focus error signal is detected by operating the difference of the sum signal of the diagonal light receiving region of the quartered light receiving part 420, and the RF signal is detected by operating the sum signal of all light receiving regions of the quartered light receiving part.
Moreover, the sub beam light receiving part 421 of the photodetector 409 is focused on the track of the recording layer of the optical disk 407, and receives the sub beam in the 3-beam method that has been reflected from the recording layer. In addition, the tracking error signal is generated by the so-called 3-beam method which uses the push-pull signal that has been operated based on the amount of received light of the main beam 422 of the quartered light receiving part 420, and the signal that has been operated based on the amount of received light of the sub beam 413 of the sub beam light receiving part 421, and tracking servo of causing the objective lens 405 to follow the track of the recording layer of the optical disk 407 is performed.
FIG. 29A to FIG. 29C are diagrams showing the configuration of the photodetector 409 of a conventional optical head.
FIG. 29A is a front view showing the configuration of the photodetector of a conventional optical head, FIG. 29B is a partial cross section of the photodetector shown in FIG. 29A, and FIG. 29C is a diagram viewing the photodetector shown in FIG. 29B from above.
As shown in FIG. 29A and FIG. 29B, the photodetector comprises a silicon semiconductor 431, a package 441, a circuit board 442 and an FPC board 445.
In FIG. 29A, the silicon semiconductor 431 includes a quartered light receiving part 420, a sub beam light receiving part 421, an operation circuit 432 and a signal output unit 433. The operation circuit 432 converts the amount of luminous flux received by the quartered light receiving part 420 and the sub beam light receiving part 421 into voltage and performs a predetermined operation. The signal output unit 433 is connected to the operation circuit 432, and outputs the voltage to become an output signal from the operation circuit 432.
As shown in FIG. 29B, a circuit board 442 is provided to the lower part of the silicon semiconductor 431. The signal output unit 433 and the pad part 443 provided to the circuit board 442 are connected via a wire bonding 446. The circuit board 442 is mounted on and fixed to the FPC board 445 by the terminal part 444 that is electrically connected to the pad part 443. Moreover, the package 441 covers the silicon semiconductor 431, the circuit board 442 and the wire bonding 446.
As shown in FIG. 29C, when considering the region of the wire bonding 446 and the formation strength of the resin package 441, the projection area of the photodetector 409 (that is, the projection area of the package 441) relative to the projection area of the silicon semiconductor 431 will increase. The size of the photodetector 409 using this kind of resin package 441 in the X direction, Y direction and Z direction will be, for example, roughly 7 mm, 5 mm and 3 mm, respectively.
FIG. 30 is a cross section showing the configuration of the peripheral portion of the photodetector 409 of a conventional optical head 400. In FIG. 30, the optical base 411 retains, for example, a semiconductor laser 401 (not shown), a diffraction grating 402 (not shown), a beam splitter 403, a collimator lens 404 and a cylindrical lens 408. Moreover, the photodetector 409 is fixed to the holder 410. In addition, the holder 410 is fixed to the optical base 411 upon positioning the photodetector 409 so that the luminous flux enters the substantial center of the quartered light receiving part 420 (not shown).
Currently, the development of a compact optical head capable of recording or reproducing information to or from a multilayer optical disk of a high recording density having two or more recording layers is anticipated. In order to realize a compact optical head capable of recording or reproducing information to or from a multilayer optical disk, it is necessary to increase a so-called lateral magnification of a detection optical system, which is the ratio of the focal point distance of the objective lens of the optical head and the focal point distance of the collimator lens. In other words, among a plurality of recording layers, it is necessary to adopt a configuration where stray light reflected off another recording layer that is different from the recording layer to which the laser beam is focused does not enter the sub beam light receiving part, and downsize the detection optical system of outward path.
An offset is generated in the tracking error signal when stray light reflected by another recording layer enters the sub beam light receiving part. Moreover, the DC level of the tracking error signal will change due to the interference of the reflected light from the recording layer on which the optical spot is being focused, and the reflected light from another recording layer, thereby considerably degrading the performance of the tracking servo, and deteriorating the recording performance and reproduction performance. In particular, since the amount of light of the sub beam is roughly 1/10 in comparison to the main beam, the tracking error signal will change considerably due to a slight change in the amount of light caused by the interference.
Thus, considered may be distancing the main beam and the sub beam. Nevertheless, by increasing the lateral magnification and distancing the main beam and the sub beam, the positions of the light receiving parts which respectively receive light will also become separated. Consequently, the area of the photodetector will increase, and it is no longer possible to simultaneously achieve the downsizing of the optical head and the improvement of the reproduction performance.
Thus, in order to simultaneously achieve the downsizing of the optical head and the improvement of the reproduction performance, considered may be increasing the so-called lateral magnification of the detection optical system, which is the ratio of the focal point position of the objective lens and the focal point distance of the collimator lens. According to this configuration, it is possible to adopt a configuration where the stray light reflected by another recording layer does not enter the sub beam light receiving part, downsize the detection optical system of return path of the optical head 400 and downsize the optical element and the photodetector, and thereby reduce the size of the optical head 400 in the height direction. Nevertheless, in order to downsize the detection optical system of return path, it is necessary to reduce the respective focal point distances of the objective lens, the collimator lens and the cylindrical lens, and downsize the respective components. Thus, it is necessary to downsize the photodetector and improve the radiation capability associated with the downsizing of the photodetector.
The power consumption of the photodetector during recording and during reproduction is, for example, approximately 0.15 W to 0.5 W. When the volume becomes approximately 1/10 due to the downsizing of the photodetector, if the power consumption is the same as before the downsizing, the temperature of the photodetector itself will increase, and far exceed the operational guaranteed temperature of the photodetector. In the case of a photodetector in a so-called slim size optical disk drive (optical information equipment), the size in the X direction is, for example, roughly 7 mm, the size in the Y direction is, for example, roughly 5 mm, and the size in the Z direction is, for example, roughly 3 mm. Note that the size of a photodetector in a slim size optical disk drive in the Y direction is desirably 4 mm or less.
When the capacity of the photodetector is caused to be 1/10 by increasing the magnification of the detection optical system of the photodetector in which the temperature rises approximately 10° C., the temperature of the photodetector will rise 20° C. to 30° C. or higher. When recording information on a multilayer optical disk requiring high recording power, and when recording information on a multilayer optical disk at a high speed, the temperature of the blue semiconductor laser as the light source will rise, and the temperature of the overall optical head will also increase. Thus, when the operating environment temperature becomes a high temperature, the temperature of the photodetector will increase even further.
FIG. 31 is a diagram explaining the temperature rise of the conventional photodetector 409. Let it be assumed that the guaranteed temperature of the photodetector is 100° C. If the peripheral temperature of the optical disk drive (optical information equipment) 450 is set to 60° C., during the high speed recording of the multilayer optical disk 407, the temperature of the photodetector 409 will reach approximately 90° C. due to the heat generation from the circuit board, the heat generation from the semiconductor laser 401, the heat generation from the objective lens actuator 406, the heat generation from the laser driver, and so on. In addition, due to the smaller thermal capacity caused by the downsizing of the photodetector 409 and aggravation of the radiation characteristics, when the temperature rises 20° C. to 30° C., the temperature of the photodetector 409 will become 110° C. to 120° C., and will considerably exceed the performance guaranteed temperature of the photodetector 409. In order to inhibit the temperature rise of the photodetector 409 to 10° C. or less, it is necessary to adopt a configuration where the heat generation from the photodetector 409 itself is efficiently radiated into the air.
FIG. 32 is a diagram explaining the relation between the magnification of the detection optical system of a conventional optical head and the pitch of the main beam and the sub beam on the photodetector, and the relation between the magnification of the detection optical system and the pitch of two sub beams on the photodetector. Table 1 is a table showing the relation of the magnification of the detection optical system and the pitch of the main beam and the sub beam on the photodetector, and the relation of the magnification of the detection optical system and the pitch of the two sub beams on the photodetector.
TABLE 1Magnification of DetectionOptical System(Lateral Magnification β)61416Spacing X (μm) of Main Beam120280320and Sub BeamSpacing Y (μm) of Sub Beams240560640
Note that the focus error signal is calculated based on following Formula (1), and the tracking error signal is calculated based on following Formula (2).Focus error signal=(A2+A4)−(A1+A3)  (1)Tracking error signal=(A3+A4)−(A1+A2)=k(B2−B1)  (2)
Note that, in foregoing Formula (1) and Formula (2), A1 to A4 represent the output of the respective light receiving regions of the quartered light receiving part 420, B1 and B2 represent the output of the respective light receiving regions of the sub beam light receiving part 421 that has been divided into two parts, and k represents the gain. The gain k is usually set to a value of roughly 1 to 5.
The lateral magnification of the detection optical system that is generally used in a conventional optical head is substantially 6 times, and, on the assumption that the pitch of the main beam and the sub beam on the optical disk is 20 μm, the pitch P of the main beam 422 and the sub beam 423 on the photodetector 409 will be 120 μm. Meanwhile, if the magnification of the detection optical system is set between 14 times and 16 times in order to reproduce a multilayer optical disk, the pitch P of the main beam 422 and the sub beam 423 on the photodetector 409 will increase to 280 μm to 320 μm, and, similarly, the pitch Q of the two sub beams 423 will increase to nearly triple. Thus, the size R of the silicon semiconductor 431 including the quartered light receiving part 420, the sub beam light receiving part 421 and the operation circuit (not shown) in the Y direction will increase. Since the size R of the silicon semiconductor 431 in the Y direction will increase, it is necessary to reduce the size of the photodetector 409 in the Y direction to be as small as possible, and simultaneously inhibit the temperature rise of the photodetector 409 itself efficiently.